In multi-color printing, such as sheet-fed offset printing, individual partial images in the various colors (e.g., yellow, cyan, magenta and black) are printed one over the other on the same sheet in respective printing units. In some cases, partial images or surfaces are also printed in separate, decorative or house colors (such as in packaging printing, high quality publicity brochures, and the like). As is well known, the quality of the printed products depends greatly upon the accuracy of the superimposition of the partial images. Ideally, the individual partial images coincide precisely with their required positions on the printed sheet. Departures from this ideal condition are recognized as register differences and referred to as register errors. Such departures degrade the printed image, but the subjective analysis of image degradation is usually insufficient to quantify the register error. But it is recognized that an inferior image is often the result of these register errors.
In order to quantify the value and controllably eliminate register errors, register marks, register symbols or other measuring elements are often printed on the sheet with the individual partial images. These enable register differences of the different partial images relative to one another to be quantitatively determined. Usually, these register marks or the like are printed in an area which is free of printed text, such as in the margins or at the corners of the printed sheet. It is then possible, from an examination of the register marks to determine register differences and thus the adjustments required for the printed plate or cylinder associated with a specific partial image.
Register symbols in the form of crosses can allow a rapid determination that there is no register difference and hence no register error. However, determination and quantification of the register difference using such register crosses and a measuring microscope or the like is tedious and time-consuming.
Tools are available to facilitate register difference determination. For example, the FOGRA venier measuring element or the "Interferenz-Vernier" described in Deutscher Drucker No. 1, 1990, pp. W27-30 provides read-off convenience and also improves read-off accuracy. It requires, however, relatively considerable space in the unprinted area in the partial image prints.
Apart from visual determination of register differences, automated measuring equipment is also available which cooperates with specially devised register marks to determine register differences, such as by photoelectric scanning of the marks. Register marks and corresponding scanning equipment of this type is described, for example, in German published patent application DE 3,709,851 A1 and DE 3,719,766 A1. The disadvantage of such equipment is that special register marks are required and become inflexible in the long term. Moreover, the scanning equipment is usable only for the register determination and is not of general applicability or useful for other purposes.
The subject of video technology with image processing for determining register differences by means of register crosses is discussed in European published application EP 177885 A2 and EP 221472 A2. Register difference determination in those systems is interactive and semi-automatic. The register crosses are recorded by a video camera and shown on an enlarged scale on a monitor activated by an operator by means of a cursor control. An evaluation computer determines register differences from the image coordinates. Just as in the case of determination with a magnifier or microscope, the accuracy of register difference determinations on these systems depends on the printed quality of the register crosses, i.e., on the contour sharpness. Minor register differences can be reliably determined only with very fine register cross lines, and accordingly the blurred structure of such fine lines due to the print and roughness of the paper has a negative impact at high magnification.
Register difference determination by use of dedicated register symbols, register marks or measuring elements printed on the sheet solely for register determination has the disadvantage that, first of all, additional space is required to accommodate the marks. Secondly, the state of coincidence of the partial images in the actual printed text (which is the key factor for the quality of the printed product) is not necessarily in coincidence with the register symbols often printed in the margins. The experienced printer, therefore, usually does not check register solely by reference to the register symbols printed in the margins of the sheet, but also by reference to a number of printed image details which are important to the print and also clearly show register differences.
Printed image details which can show register differences are, for example, colored indicia formed by superimposing at least two colored inks on a single color or unprinted background. Others are light colored indicia (e.g., unprinted areas) on a dark background which is formed by superimposing at least two colored inks (e.g., by a reverse character). Other image details particularly suited to detecting register differences are contours formed by a number of partial images such as hair lines, hands of clock, image lines or the like.
An example of a negative or reverse image referred to above is a letter formed of an unprinted area in a background produced by superimposing two or more colors such as magenta and cyan. Register differences of the cyan image with respect to the magenta then appear as correspondingly fine cyan or magenta color fringes. Color fringes due to register differences can be detected very easily in the case of long and rectilinear contours. The reason for this lies, at least in part, in the vernier visual acuity of the eye and in the color contrast of a color fringe.
If, however, register differences of this type are to be assessed on a number of key locations in the printed image, there can be required a tedious method which demands substantial experience and even then is difficult to produce quantified results. A number of measurements are required to be able to detect the distribution of the register or register differences over the printed sheet and thereby effect optimal adjustment of the register controls. Furthermore, detecting register differences by color fringes is difficult of quantification and thus it demands great experience to view an image with a color fringe and determine the precise amount of register adjustment to be applied to the press.
Published European patent application EP 127831 A2 describes register difference determination using a video camera followed by a computer unit. The position of different colored raster dots relative to one another in the image is compared with their required position. The raster dots are used almost like register symbols which, if the partial images are in the optimal relative positions, occupy specific and exactly predetermined distances one from another. A disadvantage of this approach is that the required position of the raster dots in the individual partial images relative to one another must be known. Thus, it appears that the measuring principle of this proposal is suitable primarily for assessing and determining register behavior of the partial images of an image location where they are recorded on a moving printed web. Thus, it is possible to assess the same image locations of consecutive printed images in respect of their register behavior.
In summary, it can be said of the prior art techniques which utilize printed dots, those which require an analysis on a dot-by-dot basis are not as workable as can be desired, and do not generally provide the necessary flexibility. Those systems which scan register marks are deficient in that the register marks are not within the image itself, and the register marks also present certain difficulties as noted above.